There are several industrial applications which require the level of a flowable solid material such as a granular material in a container to be maintained within predetermined limits. One example is the input chamber of a plastic molding machine, in which a constant supply of granular plastic should be maintained. Another example of an application of this sort is a silo used for grain storage. In this application there should be no filling of the silo, with grain, above a given level. Other examples are too numerous to mention.
In any such application, the input of flowable material, once initiated, should be stopped once a particular level is reached. For effective operation, the interruption should be automatic, and there should be an automated means to resume filling whenever the level of the flowable mass is reduced below some minimum. This entails a sensor to determine the level of filling in the container, and use of such determination to control fill operations.
Many such sensors have been proposed, but not all are really satisfactory. The sensor should be independent of any aspect or quality of the flowable material, and independent of the filling device or system in which the filling device is installed since the material involved may change substantially and the position of the discharge seal may be oriented incorrectly. The sensor should be simple and reliable, capable of long life, without regard to frequency of use or change of material. Further, the sensor should be readily and quickly adaptable to changes in the level requirements of the comprehensive system in which it is employed. Additionally, the sensor must be capable of operations in environments in which the temperature may be as high as 400.degree. F. Prior sensors have generally failed in one or more such respects.